Generally, dehydrated foods such as "ajitsuke-nori (a toasted and seasoned laver), "okaki (a dehydrated rice cake)", "okoshi (a millet-and-rice cake)" and cookie are enclosed in a moistureproof package such as a can, bottle, polyethylene-laminated aluminium foil package, and, in the moistureproof package, a desiccant such as silica gel or calcium oxide is used to decrease the atmospheric moisture and relative humidity in the moistureproof package so as to retain the quality of the dehydrated food.
Conventional desiccants, however, have the drawback that their use involves possible danger because they may come into contact with the skin or mucous surface membrane or may be misguidedly ingested. For this reason, development of a much safer desiccant has been strongly expected.
The moisture in foods greatly influences, in addition to their physical properties, their shelf lives. Generally, hydrous foods are susceptive to microbial contamination, as well as to alteration and deterioration such as hydrolysis, souring and browning.
As one means to decrease the moisture in foods to prolong their shelf lives, various dehydration methods have been employed: for example, "sato-zuke (preservation in sugar)" as in the case of "buntan-zuke (a candied citrus fruit buntan)", "shio-zuke (pickling in salt)" as in the case of "takuan-zuke (a pickled Japanese radish)", and drying method as in the case of "funmatsu-miso (powdered soybean paste)" or "funmatsu-kaju (fruit juice powder)".
However, sugar has the disadvantages that its excessive sweetness does not suit the recent preference; that the intake of sugar is a major factor of causing dental caries; and still that an excessive intake of sugar increases blood cholesterol. It has been pointed that an excessive intake of salt is one of the major causes of geriatric diseases such as hypertension and cancer. Thus, physicians advise to reduce salt intake as much as possible.
The drying method provides only insipid foods because vaporization inevitably disperses flavor during the processing steps.
Pharmaceuticals containing a bioactive substance, for example, lymphokine, hormone, vitamin, intact bacteria cell or antibiotic, are produced generally by heat-drying or lyophilizing the bioactive substance in the presence of a large amount of a stabilizer. This is because bioactive substance is unstable under high moisture conditions.
The stabilizers which have been used are water-soluble polymers such as albumin, casein, gelatin and hydroxylethyl starch.
Dehydration in the presence of these water-soluble polymers, however, has the demerits that it consumes a relatively large amount of energy: that it may insolubilize the final product; and still that it may inactivate bioactive substances.